Lignin - ACS Publications - American Chemical Society

Chapter 22

A Comparison of the Structural Changes Occurring in Lignin during Alcell and Kraft Pulping of Hardwoods and Softwoods 1

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Downloaded by STANFORD UNIV GREEN LIBR on June 11, 2012 | http://pubs.acs.org Publication Date: November 30, 1999 | doi: 10.1021/bk-2000-0742.ch022

Ying Liu , Sandra Carriero , Kendall Pye , and Dimitris S. Argyropoulos 1

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Department of Chemistry and PAPRICAN, Pulp and Paper Research Centre, McGill University, 3420 University Street, Montreal H3A 2A7, Quebec, Canada Repap Technologies Inc., 2650 Eisenhower Avenue, Valley Forge, PA 19482 2

During this investigation, softwood and hardwood chips were pulped to different extents using conventional kraft and Alcell protocols. The dissolved and residual lignins were then isolated and their functional group distributions examined using quantitative P N M R . For both wood species, the solubilized kraft lignins contained the highest abundance of phenolic hydroxyl groups at all degrees of delignification, while the residual kraft lignins contained the lowest. This may be related to the considerably greater solvating abilities of alkaline aqueous media as opposed to those of ethanol toward inducing solubilization of the phenolic moieties. Consequently, at a given degree of delignification, a greater proportion of phenolic units are retained in the Alcell pulps, contributing to their documented higher reactivity with oxygen. As far as residual lignins are concerned, the data showed that condensed phenolic units are formed in greater abundance within softwood than in hardwood pulps for both processes. More specifically, condensed phenolic structures were formed most rapidly in the residual lignins of the softwood Alcell pulps and in particular during the early and latter phases of delignification. The stability of carbonium ions induced under the acidic conditions and the elevated temperatures of the Alcell process, may cause the formation of stable carbon-carbon condensed structures. These condensation reactions may be responsible for the deceleration of the delignification observed for softwoods during the Alcell process. This work also verified the presence of ester linkages that link p-hydroxyphenyl units within aspen milled wood lignin. 31

Introduction The kraft pulping process has, for a long time, been the dominant method of production of strong chemical pulp for printing and writing papers. In recent years, however, kraft mills have been under public pressure to reduce or even

© 2000 American Chemical Society In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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eliminate their air and effluent discharges with mercaptans and other reduced sulfur compounds representing the majority of the odorous emissions of the process. In response to these issues a number of research efforts have been carried out aimed at the development of an economically viable and ecologically acceptable pulping process. Most alternative pulping processes that have been suggested use mixtures of organic solvents with water as the pulping media and as such they have been termed "organosolv pulping processes". Several literature accounts on this subject have reviewed the various solvent mixtures available, the resulting pulp characteristics and the overall advantages and limitations of these processes (1-4). The Alcell process is an organosolv process that uses aqueous ethanol as the pulping liquor, qualifying it as a sulfur free and environmentally benign process. This process offers a fully bleachable chemical pulp at high yields with physical and optical properties comparable to those of kraft pulp. For this process solvent recovery is feasible via distillation, offering significant capital cost savings compared to the recovery equipment essential for a kraft mill i.e. recovery boiler, lime kiln and other pollution control equipment. The lignin fraction, the major by product of the process, can potentially be utilized as either fuel or feedstock for chemical conversion (5-7). In a recent report by Lora (8), wheat straw and reed were successfully pulped by the Alcell process yielding pulps of competitive quality profiles to hardwood kraft pulps. The Alcell process, however, in its current state of development, is suitable only for the pulping of hardwoods and annual plants. Softwoods seem to be very resistant to the acidic delignification that takes place under Alcell conditions, with the precise reason remaining unclear (9). The pH of the Alcell cooking liquor is relatively low, due to the organic acids that are generated by the hydrolysis of the hemicelluloses. Therefore, in contrast to the kraft pulping process, the Alcell process is an acidic delignification process (10). Fundamental enquiries aimed at examining the underlying chemistry occurring during the Alcell process, have shown that the cleavage of ether linkages in syringyl lignins is faster than their guaiacyl counterparts present in softwood lignin (6,7). This difference was offered as an explanation for the difficulties encountered in dissolving softwood lignin out of the wood with the Alcell process. The observations of Goyal and Lora (6,7) were later confirmed by Lai (11,12) who studied the acidic delignification of aspen and Norway Spruce wood meals and demonstrated that the hydrolysis of the β-aryl ether units had a direct impact on the initial delignification rates. The β-aryl ethers present in the lignin of Norway Spruce were considerably more resistant to acid hydrolysis than those present in aspen. Under acidic conditions, lignin can be subjected to condensation reactions mainly due to the instability of the resulting C carbonium ions. Obviously such reactions will be counterproductive to delignification and may eventually offer further e

In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


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difficulties during subsequent bleaching operations. Although model compound studies by Shimada (14,5) have shown that the higher rate of lignin condensation occurring in softwood lignin models may be partially responsible for the slower softwood delignification rate, the role of these reactions during organosolv pulping remains unclear (13). In relation to these issues, a number of questions remain unanswered: Are condensation reactions of real significance during the Alcell process of softwood and hardwood? In case they do occur, are they more significant for softwoods than hardwoods and what are their formation profiles compared to the condensation reactions operating during kraft pulping? Questions of this nature as well as issues pertaining to the fundamental differences between the two processes have prompted the present investigation. In particular, softwood and hardwood chips were pulped to different extents using conventional kraft and Alcell protocols. The dissolved and residual lignins were then isolated and their functional group distributions examined using quantitative P N M R (16-25). 31

Experimental Alcell and Kraft Cooks The Alcell cooks were carried out on aspen (populus tremuloides) and black spruce (picea mariand) chips by Repap Technologies Inc. The Alcell pulping conditions were: liquor to wood ratio 4.5, alcohol concentration 46%, maximum temperature 195°C, time at maximum temperature 0-60 min. Kraft cooks were carried out on the same chips in a laboratory batch digester, using a liquor to wood ratio of 4:1, a white liquor composition of 18.0% active alkali and 30% sulphidity. Other cooking conditions were: maximum temperature 120-170 °C, time to maximum temperature 58-90 min, time at maximum temperature 0-93 min. After pulping, the pulps were thoroughly washed with water and air-dried. The pulp yields and lignin contents were determined according to standard C PPA methods. Isolation of Residual and Solubilized Kraft and Alcell Lignins Residual lignins were obtained by extraction of kraft and Alcell pulps using a slightly modified acidolysis procedure (0.1 mol/L HC1 in a 8.5:1.5 dioxane-water) for 2 hours described elsewhere (25). The extract was then concentrated, reprecipitated into a large volume of water whose pH was adjusted to 2-3. Residual lignins were dried under vacuum for 24 hours prior to quantitative P N M R analyses. 31

The lignins from the kraft and Alcell black liquors collected at the various degrees of delignification were precipitated by acidification (pH *3-4). The liquor was centrifugea, washed with acidified water until the supernatant liquor was clear. The lignins were then dissolved in a mixture of dichloro- methane.ethanol (2:1),


450 filtered and precipitated in ether, air-dried, and further dried under vacuum for 24 hours, prior to characterization. Lignin Alkaline Hydrolysis


Aspen milled wood lignin (100 mg) were dissolved in 10 mL of 2 M NaOH and stirred at 25°C for 48 hours. After the hydrolysis, the reaction mixture was acidified to pH 3 using 0.2 mol/L HC1 and centrifuged. The solid residue was washed with water, centrifuged, freeze-dried and dried to constant weight at 40°C under reduced pressure. Treatment of Aspen Milled Wood Lignin under Kraft Pulping Conditions Initially a white liquor solution was prepared containing 7.0 g of sodium hydroxide and 6.2 g of sodium sulfide (Na S.9H 0) per 100 mL deionized water. This solution was then mixed with an equal volume of absolute dioxane. Milled wood lignin (100 mg) was dissolved in 1.33 g of the above solution (approximately simulating a liquor/wood ratio of 4/1). The reaction was carried out in a 10 mL stainless steel bomb in an oil bath at 160°C for 20 min. After the reaction, the mixture was acidified to pH 3 using 0.2 mol/L HC1. The precipitate formed was collected by centrifugation, washed with acidified water, and dried to constant weight at 40°C under reduced pressure. The solution resulting from the aqueous acidic precipitation was then freeze-dried, extracted twice with tetrahydrofuran, dried over N a S 0 and evaporated under reduced pressure. The residue was then dried in a vacuum oven at 25°C. 2

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Quantitative P NMR Analyses Lignin samples were prepared by dissolving approximately 40 mg in 800 μL of a mixture of 1.6:1 pyridine/CDCl (1.6/1 v/v) at room temperature. The lignin solution was then phosphitylated with 100 μL of 2-chloro-4,4,5,5-tetramethyl1,3,2-dioxaphospholane. Solutions (100 μ ί ) of cyclohexanol (1.1 mg) and chromium acetylacetonate (0.55 mg) in 10 mL of pyridine/CDCl (1.6/1 v/v) were also added to serve as the internal standard and relaxation reagent, respectively. The quantitative P N M R spectra were acquired by inverse gated decoupling on a Varian XL-300 M H z spectrometer operating at 121.5 M H z , by methods identical with those described previously (20,23,24-25). A n observation sweep width of 6600 Hz was used. A l l chemical shifts reported were relative to the reaction product of water with the phosphitylation reagent, which has been reported to give a sharp signal at 132.2 ppm (22-24). For each spectrum 128 transients were acquired with a delay time of 25 sec between successive pulses. 3

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Results and Discussion 31

Typical quantitative P N M R spectra and signal assignments for residual and


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solubilized kraft and Alcell lignin samples isolated at approximately 40% extend of delignification are shown in Figure 1 for both the softwood and the hardwood species. The integration ranges applied and signal assignments are based on model compound and lignin studies carried out previously (16-20, 22,24). Significant differences are apparent for all functional groups between the two species and the two processes. In the following account these data will be critically examined and discussed in the light of the underlying principles and chemistry for each process and species.


Delignification Profiles The total phenolic hydroxyl group data (sum of syringyl, guaiacyl and condensed phenols) obtained for residual and solubilized lignins during the Alcell and kraft cooks for both species, as a function of the delignification extent, are plotted in Figure 2 . The delignification degree has been calculated by taking into account the pulp yields and lignin contents determined as Klason and U V lignins. The enumerated limitations of pulping softwoods under Alcell conditions become obvious in the extend of delignification data being sampled during this effort. The softwood could only be delignified up to about 60% using the Alcell process. As such the collected data is limited in the 10-60% delignification range for the same of cooking time range. With the exemption of the plot describing the phenolic group contents of the residual Alcell lignins, the total amount of phenolic groups present in all other lignins was found to increase as the degree of delignification was increased. This demonstrates that the gradual scission of aryl ether bonds present in lignin releases free phenolic hydroxyl groups, in agreement with previous accounts that have examined kraft and Alcell lignins using quantitative P - N M R (21,22,24, 25). 31

For both wood species, the solubilized kraft lignins seem to contain the highest abundance of phenolic hydroxyl groups at all degrees of delignification, while the residual kraft lignins the lowest (Figure 2). This may be related to the considerably greater solvating abilities of alkaline aqueous media as opposed to those of ethanol toward inducing solubilization of the phenolic moieties. This is also confirmed by the fact that for both wood species and at all degrees of delignification the Alcell residual lignins contained greater amounts of phenolic hydroxyl groups than the kraft residual lignins. Consequently the Alcell solubilized lignins contained lower amounts of phenolic hydroxyl groups than the kraft solubilized lignins. The fact that for both species the residual lignin present in Alcell pulps is enriched in phenolic hydroxyl groups as compared to residual kraft, may explain the observation that Alcell pulps can be oxygen delignified more extensively than their kraft counterparts (5, 26). The retained phenolic hydroxyl groups in Alcell pulps are easily attacked by molecular oxygen under



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Figure 1. Quantitative P N M R spectra and signal assignments of various lignins examined during this effort. The extend of delignification for all spectra displayed was approximately 40%.


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Softwood

Solubilized Kraft

Solubilized Alcell


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Residual Alcell

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Delignification (%)

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Solubilized Kraft

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Delignification (%) Figure 2. Plots of total phenolic hydroxyl groups as a function of extent of delignification for residual and solubilized lignins obtained after Alcell and Kraft cooks of softwood (upper) and hardwood (lower).


454 alkaline conditions, causing demethoxylation, ring opening and side chain elimination reactions (27). Formation of Condensed Phenolic Units The process of wood delignification involves the fragmentation of lignin macromolecules, which, almost invariably, is accompanied by competing condensation reactions that diminish the solubility of the residual lignin from the wood matrix (13,28). In this section the term "condensation" refers to reactions that form carbon-carbon bonds at free C and C positions of a lignin unit. Figure 3 attempts to offer a comparison of the development of condensed phenolic hydroxyl units present in solubilized lignins, as they form during kraft and Alcell pulping for softwood and hardwood species at different degrees of delignification, while Figure 4 attempts to do the same for residual lignins. With the exemption of residual hardwood lignins (Figure 4), both processes cause the rapid build up of condensed phenolic groups, indicating that such species consistently form under both acidic and alkaline delignification conditions.


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The sigmoidal nature of the plots of Figure 3 is reminiscent of the three phases of delignification that coincide with the transition points between the initial, the bulk and the final phases of delignification (29). However, the transition points are found to occur at different extents of delignification depending on the process. During Alcell pulping for softwood these three phases are observed at much lower delignification degrees than those during Alcell pulping for hardwood and kraft pulping for hardwood and softwood. It is also interesting to note that significantly more condensed structures are accumulated within softwood and hardwood solubilized kraft lignin than within their Alcell counterparts. This may be due to the fact that condensed phenolic units formed are more readily removed in alkaline than in acidic liquors. This may also be indicative of secondary intermolecular condensation reactions occurring with greater facility in homogeneous systems under kraft conditions than under Alcell conditions. As far as residual lignins are concerned, the data of Figure 4 show that condensed phenolic units form in greater abundance within softwood than in hardwood pulps. More specifically, condensed structures seem to build most rapidly in the residual lignins of the softwood Alcell pulps and in particular during the early and latter phases of delignification. It is very likely that for softwoods the majority of the condensed phenolic units formed during the initial phase of the Alcell process is retained in the pulp instead of being transported into the Alcell liquor. The stability of carbonium ions induced under the acidic conditions and the elevated temperatures of the Alcell process, may cause the formation of stable C - C and C - C , structures bearing phenolic hydroxyl groups. This is evidenced by the pronounced signals between 140-141.7 ppm in the spectra of Figure 1 (22,24). e

e


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Solubilized Lignins 2.00

0.80

^1.80 ο 1.60 -ë-1.40 § 1.20

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Kraft Softwood

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°-0.80 "D Φ

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Delignification (%) Figure 3. Plots of condensed phenolic hydroxyl groups as a function of extent of delignification for solubilized lignins obtained after Alcell and Kraft cooks of softwood and hardwood.

Figure 4. Plots of condensed phenolic hydroxyl groups as a function of extent of delignification for residual lignins obtained after Alcell and Kraft cooks of softwood and hardwood.


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In an effort to gain further insight into the profiles of formation of condensation reactions under Alcell pulping conditions, the amounts of condensed phenolic structures present within the residual lignins were correlated with the extent of delignification and the respective pulping time profiles (Figure 5). The negative values on the x-axis indicate actual reaction times below the desired temperature of the Alcell process (195°C). While aspen wood was significantly delignified (40% delignification) even before optimum temperature was reached (time 0 and beyond), almost no delignification was observed for the case of black spruce wood. At the same time, during the heat-up period and well beyond it, condensed structures were found to rapidly accumulate within residual black spruce lignins. In contrast, the condensed structures were found to accumulate at considerably lower rates within aspen residual lignins. These condensation reactions may also be responsible for the deceleration of the delignification rate observed for the softwood after 40 minutes at temperature (195°C). In contrast, the rate of delignification of the hardwood was considerably higher throughout the process. It is also interesting to observe that for softwood the sigmoidal plot observed for the accumulation of condensed phenolic structures as a function of pulping time, is in concurrency with the plot that describes its delignification profile as a function of pulping time. This behaviour can also be considered as evidence that the condensation reactions that occur in softwood inhibit its efficient delignification under Alcell pulping conditions. Meshgini and Sarkanen (30) have showed that the rate of acid hydrolysis of a-aryl ether softwood model dimers was dramatically reduced when the benzyl moiety was changed from a guaiacyl to a syringyl nucleus. This is probably due to the fact that guaiacyl carbocations formed in acidic media are more reactive than their syringyl counterparts, rapidly condensing with other electron-rich aromatic carbons. Condensation products in which the guaiacyl nuclei act as electron-rich aromatic carbons are considered to be much more stable (15). Additives of a nucleophilic nature may prevent the reactive benzyl carbonation intermediates from undergoing counterproductive intermolecular condensations with other lignin fragments during the Alcell process. The Fate of p-Hydroxyphenyl Groups Under Kraft and Alcell Pulping Conditions The occurrence of p-hydroxyphenyl units in wheat straw lignins has been widely reported (31-34). Their quantitative estimation has been made on the basis of permanganate oxidation (33) and mild hydrolysis/quantitative P N M R experiments (35). Furthermore, a number of workers have shown that aspen wood contains relatively high amounts of p-hydroxyphenylpropane units (31, 36, 37). For straw lignin such units have been considered to be mainly due to the presence of esterified p-coumaric acid (34, 35), but their bonding patterns in 31



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Time (min) Figure 5. Profiles for the formation of condensed phenolic structures present within residual lignins under Alcell pulping conditions and correlation with the rates of delignification for softwood and hardwood.


458 aspen are not yet clear. One possibility is that enzymatically pre-esterified pcoumaric acid with p-hydroxycinnamyl alcohol monomers, may cause the formation of p-hydroxycinnamyl p-coumarates which could participate in the formation of the lignin macromolecule by conventional oxidative coupling reactions to yield γ-ρ-coumaroylated lignin (38). In relation to the objectives of this effort, however, it was thought appropriate to examine the role and fate of such units under kraft and Alcell pulping conditions. As expected, the P N M R spectrum of aspen milled wood lignin (Figure 6a) shows signals characteristic of the phosphitylated hydroxyls groups * of p-hydroxyphenyls (137.4-138 ppm). On the basis of this spectrum the aspen milled wood lignin sample was determined to contain about 4 p-hydroxyphenyl groups per 100 phenyl propane units. Furthermore, this sample was subjected to mild alkaline hydrolysis resulting in complete ester hydrolysis (35, 39). After quantitative recovery its P spectrum was recorded (Figure 6b). This treatment caused a distinct decrease in the amount of p-hydroxyphenyl groups and an increase in the amount of carboxylic acids, implying that the p-hydroxyphenyl units are partly esterified within aspen milled wood lignin, in agreement with the results of previous accounts related to hardwood milled wood lignins, straw lignins and grasses (37,40,41).


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In accordance with previous accounts (21), signals responsible for phosphitylated hydroxyl groups of p-hydroxyphenyl poups were found to be present in the P N M R spectra of Alcell solubilized and residual lignins. The presence of free phydroxyphenyl groups in organosolv hardwood lignins has also been reported by Goyal and Lora on the basis of FTIR studies (7) and Gallagher on the basis of C N M R studies (42). However, only trace amounts of such species were apparent in the P N M R spectra of kraft residual lignins (Figure 1). To further clarify the role of p-hydroxyphenyl groups under pulping conditions, aspen milled wood lignin was subjected to kraft delignification conditions and the resulting liquor after acidification gave a precipitate whose P N M R spectrum (Figure 6c) was free of p-hydroxyphenyl moieties. However, the remaining aqueous fraction (Figure 6d) contained all the p-hydroxyphenyl units that were present in the original milled wood lignin (0.18 mmol/g lignin). These experiments verified the presence of ester linkages that link p-hydroxyphenyl units within aspen milled wood lignin. In addition they supplied convincing evidence that such units do not participate in alkali promoted condensation reactions during kraft pulping of aspen, since they were quantitatively recovered in the aqueous fraction. It can also be concluded that the linkages of the p-hydroxyphenyl units within aspen wood lignin are more resistant to the Alcell process than the kraft process. 3 l

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Carboxylic Acid Group Formation The formation of carboxylic acids within the polymeric matrix increases the hydrophilicity of lignin macromolecules and promotes their dissolution in the


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Internal standard

\

Condensed phenolic units

^

Syringyl Guaiacyl and demethylated phenolic units phenolic units Carboxylic acids


Aliphatic OH

ΤΤΠ (

111

» H m 148

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1111 M 146

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1111

142

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i i

111

1111 i i i i 138

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111 i 111

136

PPM

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134

Figure 6. Quantitative P N M R spectra of (a) aspen milled wood lignin; (b) aspen milled wood lignin after alkaline hydrolysis; (c) acid precipitated fraction after treating aspen milled wood lignin under Kraft pulping conditions; (d) dissolved fraction after treating aspen milled wood lignin under Kraft pulping conditions.


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460 pulping liquor. For this reason it was thought appropriate to comparatively survey the profiles of formation of these groups during Alcell and kraft processes. Figure 7 shows the carboxylic group contents of residual and solubilized lignins for the kraft and Alcell pulping of black spruce and aspen woods as a function of the extend of delignification. Apparently, the kraft pulping process introduces significantly more carboxylic acids on both solubilized and residual lignins than the Alcell process. This confirms that phenolic units in lignin are more readily oxidized by oxygen or air in alkaline media than under the acidic organosolv conditions (43). The low abundance of carboxylic acids in both residual and solubilized Alcell lignins implies that their role toward inducing solubilization in the organic media of the organosolv process could be relatively minor. Conclusions The results of this work support the general view that during kraft and Alcell pulping the cleavage of aryl ether structures constitutes the most significant lignin fragmentation reaction with concomitant formation of large amounts of phenolic hydroxylic units. While phenolic groups are easily solvated and removed by the aqueous kraft pulping liquor, the Alcell pulping liquor has a lower affinity for these structures. Consequently, at a given degree of delignification, a greater proportion of phenolic units are retained in the Alcell pulps. Lignin condensation reactions were found to be significantly more facile for spruce than their hardwood counterparts under Alcell delignification conditions. Condensed phenolic lignin structures were found to rapidly accumulate within the residual softwood lignins during the early and latter stages of the Alcell pulping process. This may greatly contribute to the slow delignification rates observed for softwoods under Alcell conditions. Aspen milled wood lignin was verified to contain relatively high amounts of phydroxyphenylpropane structures (about 4 p-hydroxyphenyl groups per 100 phenyl propane units), partly esterified. During the kraft cook these units did not participate in alkali promoted condensation reactions but were quantitatively recovered in the kraft liquor. However, the linkages of the p-hydroxylphenyl units within aspen wood lignin were found to be more resistant to Alcell conditions. Acknowledgements The suggestions of Dr. J. Bouchard ofPaprican are gratefully acknowledged. This work was supported by the Natural Sciences and Engineering Research council of Canada in the form of a Strategic Grant.



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Figure 7. Plots of carboxylic acids as a function of extend of delignification for solubilized (upper) and residual (lower) lignins obtained after Alcell and Kraft cooks of softwood and hardwood.


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Lignin - ACS Publications - American Chemical Society

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